This invention relates in general to electrophotography and, in particular, to a process for preparing electrophotoconductive imaging members having multiple layers.
In electrophotography, an electrophotographic plate, drum, belt or the like (imaging member) containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The imaging member is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly to a support, such as paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. A layered photoreceptor having separate photogenerating and charge transport layers is disclosed in U.S. Pat. No. 4,265,990. The photogenerating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer.
Degradation of image quality is encountered during extended cycling with more advanced, higher speed electrophotographic copiers, duplicators and printers. Complex, highly sophisticated higher speed duplicating and printing systems place stringent requirements on photoreceptors. These requirements impose narrow operating limits.
The numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a bet in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an anti-curl backing layer and an overcoating layer.
Suitable and economical coating methods used for applying layers in multi-layer electrophotographic imaging members include dip coating, roll coating, Meyer bar coating, bead coating, curtain flow coating and vacuum deposition. Solution coating is a preferred approach because it is more economic than vacuum coating and can be used to deposit a seamless layer.
U.S. Pat. No. 4,082,551 to Steklenski et al. discloses a process of coating multiple layers onto an insulating, polyester substrate by applying solutions having dissolved coating substance and drying each applied layer before coating a subsequent layer. The coated elements, when tested, indicate no chemical interaction between the photogenerating and conducting layers and essentially no change in electrical resistivity of the conducting layer.
U.S. Pat. No. 4,571,371 to Yashiki discloses an electrophotographic photosensitive member having a charge generating layer and a charge transport layer. A dispersion of charge generating material dissolved in solvent is applied to a cured polyamide resin layer by soaking and drying at 100.degree. C. for 10 minutes to form a charge generating layer. Subsequently, a solution containing a charge transfer material is applied to the dried charge generating layer followed by drying at 100.degree. C. for 60 minutes.
U.S. Pat. No. 4,579,801 to Yashiki discloses a process for applying a dispersion of charge generating material in a solution containing a binder resin to a suitable substrate or dried underlayer. The charge generation layer can be formed by vapor deposition. Yashiki suggests that a charge transporting material, dissolved in a solution of resin, can be applied using conventional methods to form a thin film.
U.S. Pat. No. 4,521,457 to Russell et al. discloses a process for simultaneously constraining two different coating materials, and forming on a substrate a continuous, unitary layer comprising adjacent "ribbons." Each ribbon is comprised of different materials and is in edge-to-edge contact with an adjacent ribbon. The coated ribbons are dried in two zones, one at about 57.degree. C. and another at about 135.degree. C.
U.S. Pat. No. 4,855,203 to Miyaka teaches applying charge generating layers from coating solutions comprising a resin dispersed pigment. Suitable pigments include photoconductive zinc oxide or cadmium sulfide and organic pigments such as a phthalocyanine type pigment, a polycyclic quinone type pigment, a perylene pigment, an azo type pigment and a quinacridone type pigment. Miyaka discloses suitable organic solvents for preparing a coating solution of the pigments as including alcohols such as methanol, ethanol and isopropanol; ketones such as acetone, methylethyl ketone and cyclohexanone; amides such as N,N-dimethyl formamide and N,N-dimethyl acetamide; sulfoxides such as dimethyl sulfoxide; ethers such as tetrahydrofuran, dioxane and ethylene glycol monomethyl ether; esters such as methyl acetate and ethyl acetate; aliphatic halogen hydrocarbons such as chloroform, methylene chloride, dichloroethylene, carbon tetrachloride and trichloroethylene; or aromatic compounds such as benzene, toluene, xylene, ligroin, monochlorobenzene and dichlorobenzene.
Markovics et al., co-pending U.S. application Ser. No. 07/932,150, filed Aug. 19, 1992, now U.S. Pat. No. 5,476,740 Markovics et al., U.S. Application Ser. No. 08/195,427, allowed filed Feb. 14, 1994 and Nealey et al., U.S. application Ser. No. 08/414,163, filed Mar. 31, 1995, pending disclose processes for preparing an electrophotographic imaging member including the step of forming an interphase region.
Conventional electrophotographic imaging members, having at least a charge generating layer and a charge transport layer suffer numerous disadvantages. For example, electrophotographic imaging members can suffer from poor charge acceptance and can have limited photosensitivity due to limited injection of charge generated by absorbed photons into the charge transport layer. In addition, charge transport materials may diffuse and come in contact with the conductive layer, adversely affecting the electrophotographic imaging member. Notably, devices manufactured using conventional processes have limited photoresponse.
Photoreceptors with perylene charge generating pigments, particularly benzimidazole perylene, show superior performance with extended life. The perylene containing charge generating layers can be applied by a vacuum coating process. Vacuum coated charge generating layers containing perylenes show a high photosensitivity. However, vacuum coating is expensive.
Solution coating is a more economical and convenient method of applying charge generating layers. However, perylene pigments are difficult to disperse and unstable dispersions are encountered with coating perylene pigment charge generating layers from solution. Unstable dispersions cause pigment flocculating and settling that leads to coating quality problems. Unstable dispersions are difficult to process, especially in a dip coating process. Dip coated perylene containing charge generating layers show the substantial depreciation in sensitivity described above.